docs: sync documentation with recent main merges

[aheritier]

Syncs /docs with three merged feature PRs from the last 36 hours.

PR #3305 — docs/_data/nav.yml + docs/configuration/tools/index.md
Added the session_plan toolset to the nav sidebar and to the built-in tools table. The page docs/tools/session_plan/index.md was shipped in the PR itself; only the navigation wiring was missing.

PR #3335 — docs/configuration/models/index.md
Documented the new compaction_model field: schema block, Properties Reference table row, and a "Delegating Session Compaction" section with a YAML example, a note on window-sizing behaviour, a constraint callout, and a link to the example file.

PR #3338 — docs/configuration/models/index.md
Documented the new bypass_models_gateway field: schema block, Properties Reference table row, and a "Gateway Bypass" section with a YAML example, propagation-through-router note, a security callout, a constraint callout, and a link to the example file.

PRs not requiring doc changes: #3337 (OpenRouter — shipped its own docs), #3320 (session_context — shipped its own docs), #3306 (instruction_file list — shipped its own docs), #3322 (1password caching — shipped its own docs), #3325/#3304 (docs PRs themselves), #3327/#3312/#3324/#3323 (bug fixes with no config surface), #3321/#3326/#3328–#3332 (internal refactors/tests), #3334 (CI), #3314/#3331/#3339 (CHANGELOG only), #3310 (covered by PR #3313 docs sync).